This application claims the priority of German patent document 100 28 749.2, filed Jun. 10, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for detecting shock absorber damage, particularly in a motor vehicle.
Proper maintenance of vehicles requires continuous availability of information concerning the wear condition of important vehicle components, including in particular the shock absorbers.
Depending on the shock absorber construction (for example, single-tube gas pressure shock absorbers and twin-tube shock absorbers), different damage patterns may occur, such as pitting on the piston rod, wear of the piston rod or leaks in the separating piston packing, which become noticeable by a drop of the shock absorbing action. The shock absorbing effect will decrease and the vehicle body vibrations and vehicle vibrations in the vertical direction will be absorbed to a lesser degree.
In known methods for detecting shock absorber damage, the shock absorber is either tested in the installed condition on a test stand (such as the Boge Shock Tester) or is analyzed in the removed condition on a testing machine (such as the VDA Testing Machine). These known shock absorber damage detection methods have the disadvantage that they provide no information concerning the shock absorbing characteristics during driving operation. Rather, time-consuming and cost-intensive checking is required on a special test stand; and the shock absorber may even have to be removed before the checking.
One object of the invention is to provide a method and apparatus for detection of shock absorber damage which permit analysis of the shock absorber condition during driving operation.
This and other objects and advantages are achieved by the shock absorber evaluation method and apparatus according to the invention, based on the idea of using the signal of an antilock braking system (ABS) rotational wheel speed sensor to detect shock absorber damage. According to the invention, a conclusion with respect to shock absorber condition can be drawn by analyzing the ABS rotational wheel speed signal in a particular frequency range or ranges.
According to a first embodiment, the rotational speed change Δn of the rim (high-pass-filtered ABS signal) is used for detecting shock absorber damage.
As an alternative, the radius change Δr of the tire is computed from the rotational speed signal supplied by the ABS rotational wheel speed sensor according to the following equation:
                              Δ          ⁢                                          ⁢          r                =                              v            n                    -                      r            0                                              (        1        )            wherein:    v: longitudinal vehicle velocity [m/s], if slip≈0; (preferably averaged from 4 rotational wheel speed signals)    n: rotational speed of rim [wheel/s] (ABS signal, input quantity)    r0: static tire radius [m]
From the time varying signal for the radius change Δr of the tire or the rotational speed change Δn of the rim, its auto power density spectrum ΦΔr or ΦΔn is computed. The computing of the auto power density spectrum from the tire radius change signal has the advantage that it uses a quantity which is independent of driving speed.
The relationship between the road irregularity h acting upon the wheel (and thus upon the shock absorber) and the rotational speed change of the rim or the radius change of the tire can be represented system-theoretically by means of a frequency response function GΔn or GΔr which also includes the condition of the shock absorber.
For the excitation h (that is, the road irregularity), the auto power density spectrum is given by the following equation:Φh(ω)=Φh(Ω0)·vw-1·Ω0w·ω−w  (2)wherein:    Φh(Ω0): extent of irregularity (depends on the road construction, such as cement base, asphalt, cobblestone pavement, etc.) [m3]    v: longitudinal vehicle velocity [m/s]    w: bumpiness [−], (≈2)    Ω0w: reference wavelength [m−w]    ω: timing circuit frequency [wheel/s]
(Reference is made in this respect to Mitschke, M.: “Dynamics of Motor Vehicles”, Volume B: “vibrations”, 2nd Edition, Springer Publishers, Berlin, 1984).
Within the scope of the invention, it has been found that, in the auto power density spectrum of the rotational speed of the rim determined by means of the ABS rotational wheel speed sensor or the computed radius change of the tire, there are ranges which depend on the shock absorption parameter d and ranges which are essentially independent of the shock absorption parameter d. (The shock absorption parameter d characterizes the generated shock absorbing action with respect to the relative speed of the shock absorber.) In the auto power density spectrum, according to the invention the latter represent a reference frequency range which preferably has the frequencies ω2,i in the interval of 19 to 22 Hz or of approximately 30 to 33 Hz. According to the invention, the ranges which are dependent on the shock absorption parameters represent an analysis frequency range in the auto power density spectrum, which analysis frequency range preferably has frequencies w1,i in the interval of approximately 12 to 15 Hz.
For the auto power density spectrum of the radius change of the tire ΦΔr(ω1,i,d) or of the rotational speed change of the rim ωΔn(ω1,i, d) in the analysis frequency range or in the analysis interval, the following therefore applies according to the invention:ΦΔr(ω1,i,d)=|GΔr(ω1,i,d)|2·Φh(ω1,i)  (3a) andΦΔn(ω1,i,d)=|GΔn(ω1,i,d)|2·Φh(ω1,i)  (3b)
For the auto power density spectrum of the radius change of the tire or of the rotational speed change of the rim in the reference frequency range or in the reference interval, the following therefore applies according to the invention:ΦΔr(ω2,i)=|GΔr(ω2,i)|2·Φh(ω2,i)  (4a) andΦΔn(ω2,i)=|GΔn(ω2,i)|2·Φh(ω2,i)  (4b).
By forming a quotient of equations (3a) and (4a) as well as (3b) and (4b), the following equations are obtained:
                              DSKW                      Δ            ⁢                                                  ⁢            r                          =                                            ∑                              i                =                1                            k                        ⁢                                                                                                                        G                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                                                        ω                                                      1                            ,                            i                                                                          ,                        d                                            )                                                                                        2                                                                                                                          G                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                        2                                              =                                    ∑                              i                =                1                            k                        ⁢                                                                                Φ                                          Δ                      ⁢                                                                                          ⁢                      r                                                        ⁡                                      (                                                                  ω                                                  1                          ,                          i                                                                    ,                      d                                        )                                                                                        Φ                                          Δ                      ⁢                                                                                          ⁢                      r                                                        ⁡                                      (                                          ω                                              2                        ,                        i                                                              )                                                              ·                                                (                                                            ω                                              1                        ,                        i                                                                                    ω                                              2                        ,                        i                                                                              )                                w                                                                        (                  5          ⁢          a                )                        and                                                                DSKW                      Δ            ⁢                                                  ⁢            n                          =                                            ∑                              i                =                1                            k                        ⁢                                                                                                                        G                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                                                        ω                                                      1                            ,                            i                                                                          ,                        d                                            )                                                                                        2                                                                                                                          G                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                        2                                              =                                    ∑                              i                =                1                            k                        ⁢                                                                                Φ                                          Δ                      ⁢                                                                                          ⁢                      n                                                        ⁡                                      (                                                                  ω                                                  1                          ,                          i                                                                    ,                      d                                        )                                                                                        Φ                                          Δ                      ⁢                                                                                          ⁢                      n                                                        ⁡                                      (                                          ω                                              2                        ,                        i                                                              )                                                              ·                                                (                                                            ω                                              1                        ,                        i                                                                                    ω                                              2                        ,                        i                                                                              )                                w                                                                        (                  5          ⁢          b                )            wherein w is preferably approximately 2.
According to the invention, this quotient DSKWΔr or DSKWΔn corresponds to a characteristic shock absorber damage value, which changes when the shock absorption parameter d changes (that is, the shock absorber deteriorates), because, as a result, the frequency response function GΔn or GΔr is changed or influenced. This permits the derivation of a conclusion with respect to the shock absorber condition. The signal, which corresponds to the characteristic shock absorber damage value DSKWΔr or DSKWΔn, is preferably low-pass filtered in order to obtain a smoothing.
The rotational speed signal of the rim or the computed radius change of the tire contains the useful signal which is of interest (with the recognizable resonance frequencies of the vehicle body and the rim or tire as well as the resonance frequency of the rotational speed of the rim and the belt) on the one hand, and a signal which is interfering for the analysis in the frequency range, in the form of an equal component and a drift in the signal. The equal component depends on the instantaneous driving speed, while the drift is caused by acceleration or deceleration of the vehicle. These interference effects can be eliminated, preferably by high-pass filtering of the rotational speed signal, particularly of the ABS rotational wheel speed signal, preferably with a corner frequency of 1 Hz.
In the described embodiment according to the invention, computation of the characteristic shock absorber damage value DSKWΔr or DSKWΔn is a function of the parameter w (bumpiness), which is preferably equal to 2. In a second embodiment according to the invention, a computation of the characteristic shock absorber damage value is suggested which is independent of the bumpiness, that is, of the road surface.
Here, the basic idea is to introduce a second reference interval with the frequency values ω3,i, preferably in the range of from 30 to 33 Hz, the first reference interval ω2,i in this case preferably being in the range of from 19 to 22 Hz.
The following applies with respect to the auto power density spectrum of the radius change of the tire or of the rotational speed change of the rim in a reference frequency range 2 or in the reference interval 2:ΦΔr(ω3,i)=|GΔr(ω3,i)|2·Φh(ω3,i)  (6a) andωΔn(ω3,i)=|GΔn(ω3,i)|2·Φh(ω3,i)  (6b).
Based on the above equations (2), (3) and (4), a new characteristic damage value DSKW′Δr or DSKW′Δn can now be defined.
                              DSKW                      Δ            ⁢                                                  ⁢            r                    ′                =                              ∑                          i              =              1                        k                    ⁢                      (                                                                                                                                                G                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                                              ω                                                          1                              ,                              i                                                                                ,                          d                                                )                                                                                                            G                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                              2                                                                                                                                                  G                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                            G                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      3                            ,                            i                                                                          )                                                                                                              2                                      )                                              (                  7          ⁢          a                )                                                          ⁢                  =                                    ∑                              i                =                1                            k                        ⁢                          (                                                                                                                  Φ                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                                              ω                                                          1                              ,                              i                                                                                ,                          d                                                )                                                                                                            Φ                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                              ·                                                            (                                                                        ω                                                      1                            ,                            i                                                                                                    ω                                                      2                            ,                            i                                                                                              )                                        w                                                                                                                                      Φ                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                            Φ                                                  Δ                          ⁢                                                                                                          ⁢                          r                                                                    ⁡                                              (                                                  ω                                                      3                            ,                            i                                                                          )                                                                              ·                                                            (                                                                        ω                                                      2                            ,                            i                                                                                                    ω                                                      3                            ,                            i                                                                                              )                                        w                                                              )                                                                                                    DSKW                      Δ            ⁢                                                  ⁢            n                    ′                =                              ∑                          i              =              1                        k                    ⁢                      (                                                                                                                                                G                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                                              ω                                                          1                              ,                              i                                                                                ,                          d                                                )                                                                                                            G                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                              2                                                                                                                                                  G                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                            G                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      3                            ,                            i                                                                          )                                                                                                              2                                      )                                              (                  7          ⁢          b                )                                                          ⁢                  =                                    ∑                              i                =                1                            k                        ⁢                          (                                                                                                                  Φ                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                                              ω                                                          1                              ,                              i                                                                                ,                          d                                                )                                                                                                            Φ                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                              ·                                                            (                                                                        ω                                                      1                            ,                            i                                                                                                    ω                                                      2                            ,                            i                                                                                              )                                        w                                                                                                                                      Φ                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      2                            ,                            i                                                                          )                                                                                                            Φ                                                  Δ                          ⁢                                                                                                          ⁢                          n                                                                    ⁡                                              (                                                  ω                                                      3                            ,                            i                                                                          )                                                                              ·                                                            (                                                                        ω                                                      2                            ,                            i                                                                                                    ω                                                      3                            ,                            i                                                                                              )                                        w                                                              )                                                                      wherein:    Gx(ω1,i, d) frequency response function, analysis interval, center frequency ω1, partial frequencies ω1,i are within the analysis interval;    Gx(ω2,i): frequency response function, reference interval 1, center frequency ω2, partial frequencies ω2,i are within the reference interval 1;    Gx(ω3,i): frequency response function, reference interval 2, center frequency ω3, partial frequencies ω3,i are within the reference interval 2;    Φx(ω1,i, d): auto power density spectrum radius change or rotational speed, analysis interval, center frequency ω1;    Φx(ω2,i): auto power density spectrum radius change or rotational speed, reference interval 1, center frequency ω2;    Φx(ω3,i): auto power density spectrum radius change or rotational speed, reference interval 2, center frequency ω3;    ω1,i: angular frequency (2·n·f) analysis interval;    ω2,i: angular frequency (2·n·f) reference interval 1;    ω3,i: angular frequency (2·n·f) reference interval 2;    w: bumpiness road
If the widths of the three intervals are now selected to be small, and the center frequencies ω1, ω2, and ω3 such that the following applies:
            (                        ω                      1            ,            i                                    ω                      2            ,            i                              )        ≈                            (                                    ω                              2                ,                i                                                    ω                              3                ,                i                                              )                .                                  ⁢        with            ⁢                          ⁢      i        =      1    ⁢                  ⁢    …    ⁢                  ⁢    k  a characteristic damage value DSKW′ΔR or DSKW′Δn is obtained which is independent of the bumpiness w.
The following is obtained for this characteristic damage value:
                              DSKW                      Δ            ⁢                                                  ⁢            r                    ′                =                                            ∑                              i                =                1                            k                        ⁢                          (                                                                                                                                                                G                                                      Δ                            ⁢                                                                                                                  ⁢                            r                                                                          ⁡                                                  (                                                                                    ω                                                              1                                ,                                i                                                                                      ,                            d                                                    )                                                                                                                      G                                                      Δ                            ⁢                                                                                                                  ⁢                            r                                                                          ⁡                                                  (                                                      ω                                                          2                              ,                              i                                                                                )                                                                                                                          2                                                                                                                                                                  G                                                      Δ                            ⁢                                                                                                                  ⁢                            r                                                                          ⁡                                                  (                                                      ω                                                          2                              ,                              i                                                                                )                                                                                                                      G                                                      Δ                            ⁢                                                                                                                  ⁢                            r                                                                          ⁡                                                  (                                                      ω                                                          3                              ,                              i                                                                                )                                                                                                                          2                                            )                                =                                    ∑                              i                =                1                            k                        ⁢                          (                                                                                          Φ                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                                                        ω                                                      1                            ,                            i                                                                          ,                        d                                            )                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        r                                                              ⁡                                          (                                              ω                                                  3                          ,                          i                                                                    )                                                                                  )                                                          (                  8          ⁢          a                )                                          DSKW                      Δ            ⁢                                                  ⁢            n                    ′                =                                            ∑                              i                =                1                            k                        ⁢                          (                                                                                                                                                                G                                                      Δ                            ⁢                                                                                                                  ⁢                            n                                                                          ⁡                                                  (                                                                                    ω                                                              1                                ,                                i                                                                                      ,                            d                                                    )                                                                                                                      G                                                      Δ                            ⁢                                                                                                                  ⁢                            n                                                                          ⁡                                                  (                                                      ω                                                          2                              ,                              i                                                                                )                                                                                                                          2                                                                                                                                                                  G                                                      Δ                            ⁢                                                                                                                  ⁢                            n                                                                          ⁡                                                  (                                                      ω                                                          2                              ,                              i                                                                                )                                                                                                                      G                                                      Δ                            ⁢                                                                                                                  ⁢                            n                                                                          ⁡                                                  (                                                      ω                                                          3                              ,                              i                                                                                )                                                                                                                          2                                            )                                =                                    ∑                              i                =                1                            k                        ⁢                          (                                                                                          Φ                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                                                        ω                                                      1                            ,                            i                                                                          ,                        d                                            )                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                              ω                                                  2                          ,                          i                                                                    )                                                                                                  Φ                                              Δ                        ⁢                                                                                                  ⁢                        n                                                              ⁡                                          (                                              ω                                                  3                          ,                          i                                                                    )                                                                                  )                                                          (                  8          ⁢          b                )            
Equations (5a), (5b), (8a), (8b) show that a check has to take place as to whether the respective expression in the denominator exceeds a threshold to be defined, so that no division will occur by zero or by a figure close to zero. If the denominator expression falls below this threshold, the computation is not carried out.
FIG. 11 shows the nomenclature for the frequency values ω1,i, wherein x=1, 2 or 3, in the analysis and reference range on the frequency axis.
Based on equation (5a) and equation (5b), it is illustrated that, for the characteristic shock absorber damage value DSKWΔr or DSKWΔn, a quotient is computed from two auto power density values. Since the auto power density is a measurement of the energy content of a signal for a specific frequency interval, according to the invention, by means of a suitable mounting of filters, a characteristic damage value DSKWΔr,F, DSKWΔn,F or DSKW′Δr,F, DSKW′Δn,F can be defined which is similar to the DSKWΔR, DSKWΔn or DSKW′Δr, DSKW′Δn.
The characteristic damage value DSKWΔr,F or DSKWΔn,F is based on equation (5a) or equation (5b).
The definition for the auto power density spectrum is supplied in equation (A).
                                          Φ            x                    ⁡                      (            ω            )                          =                              lim                          T              ->              ∞                                ⁢                                                                                      X                  ⁡                                      (                                          j                      ⁢                                                                                          ⁢                      ω                                        )                                                                              2                                      2              ⁢              T                                                          (        A        )            
This illustrates that the auto power density spectrum of a quantity x is equal to the squared amplitude spectrum X(jω) divided by twice the observation time T.
The amplitude of a signal in a defined frequency range can be determined by filtering with a narrowly limited band pass, and subsequent value formation of the determined maxima and minima. The amplitude of the time signal corresponds approximately to the value in the amplitude spectrum for the corresponding frequency range. Since the auto power density spectrum is determined for a time period T, a filtering takes place by means of a first low pass (forming the average over the time period). This filtering is followed by a quotient formation in order to obtain the DSKWΔr,F or DSKWΔn,F (compare FIG. 6), after further filtering by means of a second low pass (to determine a long-term trend).
Because the DSKW is a relative characteristic damage value, squaring of the approximated amplitude spectrum X(jω) and the limit value formation T→∞ will not be necessary in the computation. Furthermore, if a constant bumpiness w (preferably=2) is assumed, the right-hand parenthetical expression in equation (5) can be eliminated. (It causes only a scaling of DSKWΔr or DSKWΔn.) The described structure for a characteristic shock absorber damage value DSKWΔr,F or DSKWΔn,F is illustrated in FIG. 6.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.